A microporous polyolefin film is used for a microfiltration membrane, a separator for a battery, a separator for a capacitor, fuel cell material, or a basic material for a functional membrane in which a functional material is filled in the pores to give rise to a new function. When the microporous polyolefin film is used, among these applications, as a separator for a battery, particularly a separator for a lithium ion battery, it is required to have a high piercing strength and a high elongation modulus so as to prevent internal short circuit due to foreign bodies and the like within the battery and to prevent the separator from elongating in the flow direction when wound up to form a battery.
Furthermore, “fuse effect” and “heat resistance” securing the safety of a battery are demanded as well as general physical properties in a separator for a high capacity battery of recent years. The fuse effect is a mechanism for securing the safety of a battery by allowing the separator to melt and form a film for covering electrodes and shutoff the battery when inside of the battery is overheated by overcharged states and the like. In the case of a polyethylene microporous film, it is known that the fuse temperature, namely the temperature at which the fuse effect develops, is almost around 140° C. From a viewpoint of stopping runaway reaction inside the battery as soon as possible, the better it is supposed, the lower the fuse temperature is.
Furthermore, the separator after fused should maintain its shape and have a function of keeping the insulation between the electrodes. This is heat resistance and can be grasped by dividing into film breakage properties and thermal shrinkage properties. For example, safety evaluation standard of battery, which prescribe storing at 150° C. in an oven for 10 minutes, are provided in “Standard for Lithium Batteries” of U.S. standard UL1642 from the requirement of securing the safety of battery at 150° C. In order to achieve this safety standard, it is desirable to allow the separator to maintain its shape by, after fusing the pores to be filled up, preventing the film breakage at 150° C. or more and reducing thermal shrinkage as much as possible. When short-circuit parts where the electrodes contact with each other are caused in the batter by film breakage, thermal shrinkage, particularly thermal shrinkage in the width direction of the wound-up battery body, the battery becomes non-safe, and therefore a separator excellent in such a heat resistance is strongly demanded.
Coexistence of the film breakage resistance and low heat shrinkability are important, and heat resistance of a separator can be high when both of them are satisfied. High strength and low heat shrinkability and high film breakage resistance and low heat shrinkability are, however, incompatible characteristics.
For example, as for the viewpoint of high strength, techniques of increasing the molecular weight of polyethylene for creating highly oriented molecular state or increasing the draw ratio for achieving orientation are generally known. However, such a simple high molecular orientation may cause a high shrinkage stress in a fused state, and, as a result, lowers heat resistance.
Furthermore, several attempts to improve heat resistance while maintaining high strength are also known. For example, Patent Document 1 proposed a microporous film having a high film breakage resistance by extrusion molding polyolefin and a polymer cross-linkable therewith followed by cross-linking treatment, but, to the contrary, relaxation at a high temperature was slow and accordingly it was inappropriate due to remaining strong shrinkage stress. In addition, there was a problem in productivity because a thermal cross-linking treatment step, which incurs a long period of time, was included. Meanwhile, Patent Document 2 realized high strength by using a polyolefin having a sharp molecular weight distribution, but it was difficult to have low temperature fuse and high short-circuit ability because the contents of ultra-high-molecular-weight ingredients and low molecular weight ingredients are decreased.
Patent Document 3 discloses a microporous film composed of a polyethylene having a molecular weight of 300,000 or less and a polyethylene having a molecular weight of 1,000,000 or more. However, there is no description in Patent Document 3 about necessity of a step for imparting strength to the microporous film to be heat drawn, that is, a step of drawing the sheet under heating before removing a good solvent, and therefore, it is difficult to obtain a highly strong film. Besides, a step of fusing polyethylenes having a large difference in molecular weight needs a significant period of time and lacks in productivity.
Patent Document 4 discloses a microporous film composed of a polyolefin having a molecular weight of 1,000,000 to 15,000,000 and a polyolefin having a molecular weight of 10,000 to less than 500,000. What is specifically disclosed is, however, a microporous film composed of a polyethylene having a molecular weight of 2,000,000 and a polyethylene having a molecular weight of 350,000 with the molecular weight ratio less than 10. In addition, a microporous film of low thermal shrinkage is obtained in the production method of a microporous film by containing a specific heat-treatment step as an essential step. However, thermal shrinkage was not sufficiently prevented only by the heat-treatment step as described for a microporous film comprising a polyethylene having a molecular weight of 200,000 or less.
Similarly, Patent Document 5 discloses a microporous film containing 1% or more of an ultra-high-molecular-weight polyethylene having a molecular weight of 700,000 or more and composed of a polyolefin composition having a molecular weight distribution of 10 to 300. What is specifically disclosed is, however, a composition composed of a polyethylene having a molecular weight of 2,500,000 and a polyethylene having a molecular weight of 240,000, and does not contain a low molecular weight ingredient having a molecular weight of 10,000 to 200,000 which is essential to the present invention and it is insufficient in fuse property and relaxation rate.                [Patent Document 1] JP-A-2001-181435        [Patent Document 2] Japanese Patent No. 3,351,940        [Patent Document 3] JP-A-02-21559        [Patent Document 4] JP-A-2003-3006        [Patent Document 5] Japanese Patent No. 2,711,633        